Co(ii)tetramethoxyphenylporphyrin additive to pfsa pems for improved fuel cell durability

ABSTRACT

An ion conducting membrane for fuel cell applications includes an ion conducting polymer and a porphyrin-containing compound at least partially dispersed within the ion conducting polymer. The ion conducting membranes exhibit improved performance over membranes not incorporating such porphyrin-containing compounds.

TECHNICAL FIELD

The present invention relates to ion conducting membranes for fuel cellapplications.

BACKGROUND

Fuel cells are used as an electrical power source in many applications.In particular, fuel cells are proposed for use in automobiles to replaceinternal combustion engines. A commonly used fuel cell design uses asolid polymer electrolyte (“SPE”) membrane or proton exchange membrane(“PEM”) to provide ion transport between the anode and cathode.

In proton exchange membrane type fuel cells, hydrogen is supplied to theanode as fuel and oxygen is supplied to the cathode as the oxidant. Theoxygen can either be in pure form (O₂) or air (a mixture of O₂ and N₂).PEM fuel cells typically have a membrane electrode assembly (“MEA”) inwhich a solid polymer membrane has an anode catalyst on one face, and acathode catalyst on the opposite face. The anode and cathode layers of atypical PEM fuel cell are formed of porous conductive materials, such aswoven graphite, graphitized sheets, or carbon paper to enable the fuelto disperse over the surface of the membrane facing the fuel supplyelectrode. Each electrode has finely divided catalyst particles (forexample, platinum particles), supported on carbon particles, to promoteoxidation of hydrogen at the anode and reduction of oxygen at thecathode. Protons flow from the anode through the ion conductive polymermembrane to the cathode where they combine with oxygen to form waterwhich is discharged from the cell. Typically, the ion conductive polymermembrane includes a perfluorinated sulfonic acid (PFSA) ionomer.

The MEA is sandwiched between a pair of porous gas diffusion layers(“GDL”), which in turn are sandwiched between a pair of non-porous,electrically conductive elements or plates. The plates function ascurrent collectors for the anode and the cathode, and containappropriate channels and openings formed therein for distributing thefuel cell's gaseous reactants over the surface of respective anode andcathode catalysts. In order to produce electricity efficiently, thepolymer electrolyte membrane of a PEM fuel cell must be thin, chemicallystable, proton transmissive, non-electrically conductive and gasimpermeable. In typical applications, fuel cells are provided in arraysof many individual fuel cell stacks in order to provide high levels ofelectrical power.

One mechanism by which ion conducting polymer membranes degrade is vialoss of fluorine (i.e., fluoride emission) under open circuit voltage(OCV) and dry operating conditions at elevated temperatures. Additivesto PFSA membranes are required to improve fuel cell life, increasemembrane durability and reduce fluoride emissions under theseconditions.

Accordingly, there is a need for improved ion conducting membranes withreduced fluoride emissions.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The present invention solves one or more problems of the prior art byproviding in at least one embodiment an ion conducting membrane for fuelcell applications. The ion conducting membrane of this embodimentincludes an ion conducting polymer and a porphyrin-containing compoundat least partially dispersed within the ion conducting polymer in asufficient amount to reduce fluoride emissions from the membrane.Moreover, the incorporation of a porphyrin-containing compoundadvantageously increases membrane life while decreasing electrodevoltage degradation in fuel cells operating at open circuit conditionsat 95° C. and 50% relative humidity. Additional benefits include reducedcost compared with additives presently used to mitigate PFSA-fuel cellion conducting membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 is a schematic illustration of a fuel cell that incorporates anion conducting membrane of one or more embodiments of the invention;

FIG. 2 provides plots of the cell voltage degradation and fluoriderelease rate (FRR) versus time for Nafion® 1000 membrane with andwithout Cobalt(II)tetramethoxyphenylporphyrin; and

FIG. 3 provides plots of the relative humidity (“RH”) sweep profile forNafion® 1000 with and without Cobalt(II)tetramethoxyphenylporphyrin.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made in detail to presently preferredcompositions, embodiments and methods of the present invention, whichconstitute the best modes of practicing the invention presently known tothe inventors. The Figures are not necessarily to scale. However, it isto be understood that the disclosed embodiments are merely exemplary ofthe invention that may be embodied in various and alternative forms.Therefore, specific details disclosed herein are not to be interpretedas limiting, but merely as a representative basis for any aspect of theinvention and/or as a representative basis for teaching one skilled inthe art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, allnumerical quantities in this description indicating amounts of materialor conditions of reaction and/or use are to be understood as modified bythe word “about” in describing the broadest scope of the invention.Practice within the numerical limits stated is generally preferred.Also, unless expressly stated to the contrary: percent, “parts of,” andratio values are by weight; the description of a group or class ofmaterials as suitable or preferred for a given purpose in connectionwith the invention implies that mixtures of any two or more of themembers of the group or class are equally suitable or preferred;description of constituents in chemical terms refers to the constituentsat the time of addition to any combination specified in the description,and does not necessarily preclude chemical interactions among theconstituents of a mixture once mixed; the first definition of an acronymor other abbreviation applies to all subsequent uses herein of the sameabbreviation and applies mutatis mutandis to normal grammaticalvariations of the initially defined abbreviation; and, unless expresslystated to the contrary, measurement of a property is determined by thesame technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to thespecific embodiments and methods described below, as specific componentsand/or conditions may, of course, vary. Furthermore, the terminologyused herein is used only for the purpose of describing particularembodiments of the present invention and is not intended to be limitingin any way.

It must also be noted that, as used in the specification and theappended claims, the singular form “a,” “an,” and “the” comprise pluralreferents unless the context clearly indicates otherwise. For example,reference to a component in the singular is intended to comprise aplurality of components.

Throughout this application, where publications are referenced, thedisclosures of these publications in their entireties are herebyincorporated by reference into this application to more fully describethe state of the art to which this invention pertains.

With reference to FIG. 1, a fuel cell that incorporates an ionconducting membrane of one or more embodiments of the invention isprovided. PEM fuel cell 10 includes polymeric ion conductive membrane 12disposed between cathode catalyst layer 14 and anode catalyst layer 16.Polymeric ion conductive membrane 12 includes an effective amount ofstannate as set forth below. Fuel cell 10 also includes conductiveplates 20, 22, gas channels 60 and 66, and gas diffusion layers 24 and26.

In an embodiment of the present invention, an ion conducting membranefor fuel cell applications includes an ion conducting polymer and aporphyrin-containing compound at least partially dispersed within theion conducting polymer. In a variation, the porphyrin-containingcompound the porphyrin-containing compound includes a moiety havingformula 1:

wherein:

R₁, R₂, R₃, R₄, R₅, R₆, R₇, R₈, R₉, R₁₀, R₁₁, R₁₂ are each independentlyhydrogen, alkyl, or aryl. In a refinement, R₁, R₂, R₃, R₄, are eachindependently substituted or unsubstituted alkyl or phenyl. In anotherrefinement, R₁, R₂, R₃, R₄, are each phenylmethoxy. In still anotherrefinement, R₅, R₆, R₇, R₈, R₉, R₁₀, R₁₁, R₁₂ are each hydrogen. In thiscontext, substitutions may be with halogens, methoxy, ethoxy, and thelike. In addition, in the case of aryl and phenyl, substitutions mayalso be with alkyl groups.

In another variation of the present embodiment, the porphyrin-containingcompound is present in an amount from about 0.001 to about 50 weightpercent of the total weight percent of the total weight of the ionconducting membrane. In a refinement, the porphyrin-containing compoundis present in an amount from about 0.1 to about 10 weight percent of thetotal weight percent of the total weight of the ion conducting membrane.

In still another variation of the present embodiment, theporphyrin-containing compound has formula 2:

wherein M is a metal or metal-containing moiety. Examples of suitablemetals for M or for inclusion, include but are not limite do in themetal containing moiety include Co, Fe, Mg, Mn, Cu, Ni, Pd, Ru, Vn, Zn,Al, B, Si, Al, In, Pb, Ag, Sn, Ti, V, Pt, Ce, and the like. Specificexamples for M include Co²⁺, Co³⁺, Fe²⁺, Fe³⁺, Mg¹⁺, Mg²⁺, Mn¹⁺, Mn²⁺,Mn³⁺, ClMn³⁺, HOMn³⁺, Cu⁺¹, Cu²⁺, Ni¹⁺, Ni²⁺, Pd¹⁺, Pd²⁺, Ru¹⁺, Ru²⁺,Ru⁴⁺, Vn⁴⁺, Zn¹⁺, Zn²⁺, Al³⁺, B, Si(OH)₂ ²⁺, Al³⁺, HOIn³⁺, HOIn³⁺, Pb²⁺,Ag⁺, Sn²⁺, Sn⁴⁺, Ti³⁺, Ti⁴⁺, VO⁺, Pt²⁺, Ce³⁺, Ce⁴⁺.

In another variation of the present embodiment, the ion-conductingmembrane further comprises a metal-containing compound having a metal(i.e., metal ion) selected from the group consisting of Ce(III), Ce(IV),Mn(II) and Mn(IV). Examples of metal-containing compounds include MnO₂,Mn₂O₃, MnCl₂, MnSO₄, CeCl₃, Ce₂(CO₃)₃, CeF₃, Ce₂O₃, CeO₂, Ce(SO₄)₂)Ce(OSO₂CF₃)₃, and combinations thereof. In a further refinement, themetal-containing compound is selected from the group consisting of MnO₂,Mn₂O₃ MnCl₂, MnSO₄, and combinations thereof.

As set forth above, the membrane of the present invention includes anion conducting polymer. Such polymers include sulfonatedtetrafluoroethylene-based fluoropolymer-copolymers. Sometimes this classof polymers is referred to as perfluorosulfonic acid (PFSA) polymers.Specific examples of such polymers include the Nafion® line of polymerscommercially available from E. I. du Pont de Nemours and Company. Inanother refinement, the ion conducting polymer comprises aperfluorocyclobutyl moiety. Examples of these suitable polymers are setforth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,718,627; 2,393,967;2,559,752; 2,593,583; 3,770,567; 2,251,660; U.S. Pat. Pub. No.20070099054; U.S. patent application Ser. No. 12/197,530 filed Aug. 25,2008; Ser. No. 12/197,537 filed Aug. 25, 2008; Ser. No. 12/197,545 filedAug. 25, 2008; and Ser. No. 12/197,704 filed Aug. 25, 2008; the entiredisclosures of which are hereby incorporated by reference.

The following examples illustrate the various embodiments of the presentinvention. Those skilled in the art will recognize many variations thatare within the spirit of the present invention and scope of the claims.

Membrane Preparation. Cobalt(II)tetramethoxyphenylporphyrin (CoTMPP) isadded at 5 wt. % based on perfluorosulfonic acid (PFSA) polymer solidsin 1-propanol and water (3/2 weight ratio) and is homogenized with anIKA homogenizer for about 4 minutes. The CoTMPP is soluble in theionomer solution, which is centrifuged to remove suspended air bubbles.The blood-red supernate is coated on glass with an 8-mil coating gap,Bird applicator, and the resultant wet film is then heated at 80° C. for1 h in air and then at 130° C. for 4 h under vacuum. The green film isfloated off glass and air-dried to obtain a 16-um membrane.

The improved chemical durability is measured under open circuit voltage(OCV) using 50-cm² membrane electrode assemblies that are sub-gasketedto 38-cm² active areas. The electrode is a platinum on carbon slurrycoated on the microporous layer of a carbon fiber diffusion media to a0.4 mg Pt cm⁻² loading on both the anode and cathode. The fuel cell,with a serpentine flow-field, is operated at 95° C. and at 50% inlet R.Hat a 5/5 stoichiometry on the anode and cathode at a 0.2 A cm⁻²equivalent flow and 50 kPa gauge. The OCV is monitored for 200 to 800hours and the fluoride release rate (FRR) is measured from the analysisof outlet water to determine the percent loss of fluorine versus thetotal fluorine inventory of the membrane. The improved chemicaldurability is measured under open circuit voltage (OCV) using 50-cm²membrane electrode assemblies that are sub-gasketed to 38-cm² activeareas. The electrode is a platinum on carbon slurry coated on themicroporous layer of a carbon fiber diffusion media to a 0.4 mg Pt cm⁻²loadings on both the anode and cathode. The fuel cell, with a serpentineflow-field, is operated at 95° C. and at 50% inlet R.H at a 5/5stoichiometry of anode and cathode at 0.2 A cm⁻² equivalent flow and 50kPa gauge. The OCV is monitored for 200 to 800 hours and the fluoriderelease rate (FRR) is measured from the analysis of outlet water todetermine the percent loss of fluorine versus the total fluorineinventory of the membrane. FIG. 2 provides plots which outline theeffect of small amounts of CoTMPP to dramatically improve the OCVdurability of fuel cell membranes as compared to a comparative exampleof Nafion 1000 baseline without the added CoTMPP. The fluoride releaserate of the CoTMPP membrane is 10² lower and the degradation rate isreduced.

The fuel cell performance is measured using a 50-cm² membrane electrodeassemblies prepared by loose laying a catalyst coated diffusion mediumwith 0.4 mg Pt cm⁻² loadings on both the anode and cathode. The fuelcell, is operated from 0 to 1.5 A/cm² at 80° C. with 32% inlet R.H at aconstant 1.5/2 stoichiometry of anode and cathode at 50 kPa gauge. Theaddition of CoTMPP to proton exchange membranes does not reduce thein-situ fuel cell performance as shown in FIG. 3 which outlines theachieved fuel cell performance of >0.6 V at 1.5 A/cm² with no change inthe high frequency resistance (HFR) of the membrane owing to no lossesin membrane resistance.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1-13. (canceled)
 14. An ion conducting membrane for fuel cellapplications, the ion conducting membrane comprising: ion conductingpolymer comprises a perfluorocyclobutyl moiety; and a compound havingformula 1:

wherein: R₁, R₂, R₃, R₄, R₅, R₆, R₇, R₈, R₉, R₁₀, R₁₁, R₁₂ are eachindependently hydrogen, alkyl, aryl.
 15. The ion conducting membrane ofclaim 14 wherein R₁, R₂, R₃, R₄, are each independently substituted orunsubstituted alkyl or phenyl.
 16. The ion conducting membrane of claim14 wherein R₁, R₂, R₃, R₄, are each phenylmethoxy.
 17. The ionconducting membrane of claim 14 wherein R₅, R₆, R₇, R₈, R₉, R₁₀, R₁₁,R₁₂ are each hydrogen.
 18. The ion conducting membrane of claim 14wherein the porphyrin-containing compound has formula 2:

wherein M is a metal.
 19. The ion conducting membrane of claim 14wherein M is Co²⁺, Co³⁺, Fe²⁺, Fe³⁺, Mg¹⁺, Mg²⁺, Mn¹⁺, Mn²⁺, Mn³⁺,ClMn³⁺, HOMn³⁺, Cu⁺¹, Cu²⁺, Ni¹⁺, Ni²⁺, Pd¹⁺, Pd²⁺, Ru¹⁺, Ru²⁺, Ru⁴⁺,Vn⁴⁺, Zn¹⁺, Zn²⁺, Al³⁺, B, Si(OH)₂ ²⁺, Al³⁺, HOIn³⁺, HOIn³⁺, Pb²⁺, Ag⁺,Sn²⁺, Sn⁴⁺, Ti³⁺, Ti⁴⁺, VO⁺, Pt²⁺, Ce³⁺, Ce⁴⁺.
 20. The ion conductingmembrane of claim 14 wherein the porphyrin-containing compound isCo(II)tetramethoxyphenylporphyrin.